In conventional primary smelters, silver is removed from softened lead bullion by conventional Parkes and Williams processes. Each of these processes involves the subjection of the molten soft lead bullion to zinc additions to form a zinc/silver-rich crust with subsequent distillation in a retort separating the zinc from the silver and lead contained in the crust. Silver is subsequently separated from the lead in the crust by the process of cupellation. These processes involve temperatures of about 1200.degree. C., the use of specialized furnace systems, and are relatively slow, requiring, for example, 12 to 14 hours for treatment of a one-ton charge of zinc/silver crust. Resultingly, these processes are relatively expensive in the recovery of silver.
The Betts electrolytic process is another process also commonly used for the separation and recovery of silver and bismuth from the lead bullion. This process involves anodic dissolution of the lead into the electrolyte with accompanying concentration of silver and bismuth on the residual anode as a slime layer. Pure lead is cathodically deposited and the silver and bismuth are recovered from the anode slimes by recovery processes well known to those skilled in the art.
One early patent disclosing the separation of silver from lead utilizing zinc is that of Webster et al U.S. Pat. No. 147,454. Another U.S. patent disclosing a process for removing silver from lead alloys is that of William Morrison No. 890,160. Still another such U.S. patent is that of Jesse O. Betterton et al No. 2,062,116.
In U.S. Pat. No. 1,853,535, Jesse O. Betterton discloses a metal refining process wherein the lead bullion is treated with zinc to remove silver and subsequently with a calcium lead alloy and zinc to remove bismuth.
Another prior patent disclosing the use of calcium or a calcium-containing substance in treating lead dore containing 12 to 14% antimony, 14 to 17.4 oz/ton of silver and 0.38 oz/ton of gold to remove metal values is that of J. C. Reinhardt et al U.S. Pat. No. 2,359,718 wherein sequential heating and cooling steps are effected, requiring the temperature to be varied from about 500.degree. F. to 950.degree. F.
Guillaume J. Kroll discloses, in U.S. Pat. No. 1,428,041, a process for separating metallic constituents from metal alloys wherein alkaline-earth metals, especially calcium, are introduced into the metal alloys.
G. W. Thompson discloses, in U.S. Pat. No. 1,740,752, a process for purifying lead alloys having an overlying interface of caustic alkali containing an oxidizing agent.